Design-time package fails to build - File not found: 'Graphics.dcu'

In Delphi XE2, I have a single control in a pair of design/run time packages. Originally, everything was working fine. I've built each of them many times already. Suddenly without warning, the design time package started complaining in one of my units that the `Graphics` unit is missing when I build. `Graphics` is a standard unit, but it's not found from this one place. I haven't even made any changes to this unit, and as far as I know, any changes that could affect this. Here's just the uses at the top (interface) of this unit: ``` uses Graphics, ColorConv, Classes, Dialogs, ZLib; ``` As you can see, it's a very simple unit, and I only make very simple changes to this project. What could make it start complaining about this out of nowhere? The strange thing is that it all works fine if I install it, it just doesn't build. There's quite a bit of code, and I'd hate to have to post the entire thing.